It is already known from published specification DE 19744465 [U.S. Pat. No. 6,124,726] that there is a very substantial interest in monitoring tap changers during the entire period of a load changeover so as to thereby be able to ensure correct functioning. This is usually realized by detection of the torque plot, positional detection of the respective instantaneous setting of the tap changer and comparison of the ascertained value pairs with previously stored values. The torque plot is in that case ascertained in each load changeover with the help of the effective values of current and voltage by way of the effective power of the drive.
Usually an electric motor drives the tap changer by way of a load transmission, a bevel-gear transmission and a worm transmission. These transmissions are connected together by shafts and have different translation ratios and levels of efficiency. The two factors can influence the measurements of torque. In the case of a belt transmission, for example, slipping of the belt can occur at high temperatures, as a result of which efficiency is reduced. Even low temperatures have a significant influence on the torque plot. FIG. 1 shows a simplified plot of a first switching process at −20° Celsius. Time t is recorded on the illustrated abscissa and the ordinate depicts torque M. The time instant A here characterizes the start of the switching process. As can be clearly seen, torque initially rises very strongly. The reasons for that are, inter alia, the belt, which has low-temperature toughness, in the load transmission and the viscous lubricant in the worm and bevel-gear transmissions. After overcoming these first resistances the plot of the torque reduces until the time instant B. The second rise, between the time instants B and C, is correlated with actuation of the energy store. Just prior to release this needs more energy in order to stress the springs. At the time instant C the switching process has ended. Whereas the second rise caused by the energy store, is characteristic for the torque plot of a changeover, the first rise—between the time instants A and B—is always temperature-dependent and thus non-constant and not calculable.
It is disadvantageous with the prior art that the three transmissions also influence measurement error. These are determined by the products of the individual efficiencies and translation ratios and can in part be very substantial. This applies particularly to the first switching processes after a longer period of standstill of the entire drive train.